HEPATIC CLEARANCE Q x CA Q x CV Q(CA - CV) 1. Mass Balance

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HEPATIC CLEARANCE Q x CA Q x CV Q(CA - CV) 1. Mass Balance
Rate of Extraction

HEPATIC CLEARANCE 1 E 1 - E Extraction Ratio
2. Mass Balance Normalized to Rate of Entry 1 1 - E E Extraction Ratio

HEPATIC CLEARANCE Q Q x E Q(1 – E) Clearance
3. Mass Balance Normalized to CA Q Q(1 – E) Q x E Clearance

Recall that CL = QE; hence
CLH = QHE CLH = hepatic clearance QH = hepatic blood flow E = hepatic extraction ratio QH - from 1.0 to 1.5 L/min E – ranges from 0 to 1

Recall that CL = QE; hence
CLH = QHE Thus, if the non-renal clearance of a drug exceeds QH, some form of non-renal and non-hepatic elimination must take place.

Example: (labetalol) CLT = L/min CLR = 0.08 L/min Vss = 685 L t1/2 = 6 hr Can the nonrenal clearance be attributed solely to hepatic elimination?

CLH = QHE Suggests that CLH ~ QH Actually h QH = i E

VENOUS EQUILIBRIUM MODEL
QCin QCout Elimination

SINUSOIDAL MODEL QCin QCout Elimination

Intrinsic hepatic clearance: The ability of the liver to remove xenobiotic from the blood in the absence of other confounding factors (e.q., QH).

Since CLH = QHE

Quantitative Assessment of Hepatic Clearance Assuming the Venous Equilibrium Model

Limits:

Limits:

Limits:

Limits:

Compounds with a high CLint, are said to exhibit perfusion rate-limited elimination.

From: Rowland M, Tozer TN
From: Rowland M, Tozer TN. Clinical Pharmacokinetics: Concepts and Applications, 3rd edition, 1995, p. 162

HEPATIC EXTRACTION RATIO OF REPRESENTATIVE DRUGS
Low (<0.3) Antipyrine Diazepam Phenylbutazone Theophylline Tolbutamide Warfarin High (>0.7) Lidocaine Meperidine Propoxyphene Propranolol Verapamil Intermediate: Quinidine

CLint = 7.0 L/min, QH = 1.0 L/min Ko = 1.0 mg/min
Consider the case of administering a constant infusion of a drug with the following conditions: CLint = 7.0 L/min, QH = 1.0 L/min Ko = 1.0 mg/min Assuming drug is eliminated completely via hepatic metabolism, what would the steady-state concentration be?

What if the pt developed CHF resulting in a 25% reduction in QH?

CLint = 0.01 L/min, QH = 1.0 L/min K0 = 0.1 mg/min
In contrast, suppose we administer a low CLint? CLint = 0.01 L/min, QH = 1.0 L/min K0 = 0.1 mg/min

What if QH were reduced by 25%

FIRST-PASS EFFECT LIVER BODY HEART GUT

LIVER BODY HEART GUT

LIVER BODY HEART GUT

LIVER BODY HEART GUT

LIVER BODY HEART GUT

LIVER BODY HEART GUT

If a drug is completely absorbed after oral administration, the fraction of the oral dose that reaches the systemic circulation (F) is given as F = 1 - E Remembering that

QH >> CLint, Fg1 QH << CLint, Fg0

Determination of CLint after oral dosing:
Assumes: Drug is completely absorbed No extrahepatic metabolism System is stationary

Determination of CLint after oral dosing:

Determination of CLint after oral dosing:

Estimated in this fashion the CLint is often referred to as the oral clearance (CLo)
For a high CLint drug:

Estimated in this fashion the CLint is often referred to as the oral clearance (CLo)
For a low CLint drug:

Consider a high clearance drug (e. g
Consider a high clearance drug (e.g., propranolol) after oral and IV dosing: CLint = 10 L/min, QH = 1.5 L/min

What if drug were administered orally under these conditions?
What would happen to CLH is QH were reduced to 1.0 L/min? What if drug were administered orally under these conditions?

How would a decrease in QH affect this?

But what happens to AUC? For control conditions: When QH is decreased:

IMPACT OF PROTEIN BINDING
CLint = fubCLuint

Effect of protein binding on warfarin clearance in the rat
Effect of protein binding on warfarin clearance in the rat . Data from Yacobi A, Levy G. J Pharm Sci 64:1660, 1975.

Effect of protein binding on warfarin clearance in humans
Effect of protein binding on warfarin clearance in humans. Data from Routledge et al. Br J Clin Pharmacol 8:243, 1979.

CLuint = 0.25 L/min QH = 1.5 L/min
fub = K0 = 0.25 mg/min

CLuint = 0.25 L/min QH = 1.5 L/min
fub = K0 = 0.25 mg/min

What if fub = 0.2?

What if fub = 0.2?

What happens to free concentration?

Consider a high clearance drug iv:
CLuint = 30 L/min QH = 1.5 L/min fub = K0 = 2 mg/min

Consider a high clearance drug iv:
CLuint = 30 L/min QH = 1.5 L/min fub = K0 = 2 mg/min

What if fub = 0.5?

What if fub = 0.5?

What if fub = 0.5?

What if fub = 0.5?

Effect of displacement (D) of plasma protein binding on total and unbound concentrations of drug
From: Rowland M, Tozer TN. Clinical Pharmacokinetics – Concepts and Applications, 3rd edition

From: Rowland M, Tozer TN
From: Rowland M, Tozer TN. Clinical Pharmacokinetics – Concepts and Applications, 3rd edition

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
1.0 E CLint L/min CLH L/min 0.18 0.334 L/min 0.273 L/min Blood Concentration 0.1 Time

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
1.0 Blood Concentration 0.1 Time

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
1.0 E CLint L/min CLH L/min 0.95 27.0 L/min 1.42 L/min Blood Concentration 0.1 Time

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
0.1 Blood Concentration 0.01 Time

Effect of rifampin on the kinetics of warfarin after a single dose before (open circles) and after (closed circles) treatment with rifampin 600 mg/day. From: O’Reilly RA. Interaction of sodium warfarin and rifampin. Ann Intern Med 81:337-40, 1974.

Effect of pentobarbital on alprenolol disposition
Effect of pentobarbital on alprenolol disposition. Alprenolol was administered before (O, closed, iv, open oral) and after 10 d of pentobarbital (D). Reproduced from Alvan G, et al. Clin Pharmacol Ther 22: , 1977.

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
1.0 E QH L/min CLH L/min 0.18 0.75 L/min 0.135 L/min Blood Concentration 0.1 Time

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
1.0 Blood Concentration 0.1 Time

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
1.0 E QH L/min CLH L/min 0.95 0.75 L/min 0.71 L/min Blood Concentration 0.1 Time

Effect of changes in the determinants of hepatic clearance on the concentration versus time curves
0.1 Blood Concentration 0.01 Time

Propoxyphene’s CNS depression is enhanced when co-administered with ethanol. To assess the contribution of a pharmacokinetic interaction to this enhancement, propoxyphene was administered iv and po, with and without ethanol. The curves below illustrate the results. What is the mechanism of interaction? Propoxyphene alone Propoxyphene + ETOH Log [Propoxyphene] Log [Propoxyphene] TIME TIME

CLH AUCo fup fup Complete the following graphs for a drug with a
CLH = 20 mL/min/kg and one with a CLH = 1 mL/min/kg CLH AUCo fup fup

Complete the following graphs for a drug with a CLH = 20 mL/min/kg and one with a CLH = 1 mL/min/kg
CLuint QH fup

Complete the following graphs for a drug with a CLH = 20 mL/min/kg and one with a CLH = 1 mL/min/kg
CLuint fup

Complete the following graphs for a drug with a CLH = 20 mL/min/kg and one with a CLH = 1 mL/min/kg
QH

E QH fub fut CLT Vss t1/2 AUCo
High h n n High n i n High n n h

E QH fub fut CLT Vss t1/2 AUCo
Low h n n Low n n h Low n n h n

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